The present invention relates generally to carbonaceous materials and the products obtained therefrom, and particularly relates to the large-sized composite carbonaceous products prepared by mutually joining the carbonaceous materials together, the process for producing thereof, the above-mentioned composite carbonaceous products for use in electrode substrate of fuel cells and the process for producing the same.
More in detail, the present invention relates to the carbonaceous product comprising the carbonaeeous materials and the flexible graphite sheet interposed between the carbonaceous materials, the carbonaceous materials and the flexible graphite sheet having been joined together and integrated by calcination in an inert atmosphere as one carbon body, the joining surface of at least one of the carbonaceous materials comprising both joining parts and non-joining parts which have an optional shape and uniformly arranged on the joining surface, and the part of the flexible graphite sheet corresponding to the above-mentioned non-joining part having been deleted or not deleted, the process for producing the same, the above-mentioned composite carbonaceous products for use in electrode substrate of fuel cells and the process for producing the same.
The present invention still more relates to the carbonaceous product as the electrode substrate of fuel cells, which product has a united one body-structure comprising a carbonaceous material for the separator, the flexible graphite sheets disposed outside the separator, a plurality of carbonaceous protuberances disposed outside the flexible graphite sheets and the porous carbonaceous flat plates disposed outside the protuberances, wherein the passage of the gaseous reactants is formed by the combination of (1) the flexible graphite sheet or the flexible graphite sheet and the carbonaceous material for the separator, (2) a plurality of carbonaceous protuberances and (3) the porous carbonaceous flat plate, and the process for producing the same.
In the carbon product according to the present invention, the joining surface of the carbonaceous materials, which is joined to the flexible graphite sheet, is fabricated to have the uniformly arranged joining parts and non-joining parts with an optional shape, thereby preventing the focussing of stresses on the joining surface.
By such a fabrication, large-sized products can be produced and since the product has been made to be one body in the carbonized state, the electric- and thermal resistances of the product are small and the strength of the product is large.
Accordingly, the fields in which the most of these specific properties can be made are broad such as in the electrode substrate for a fuel cell, etc.
In recent years, carbonaceous molded articles comprising carbonaceous materials such as carbon fibers, carbon particles, etc. as the basic material have been used in various industrial fields, and with the technical progress and increase of demands, requirements of higher order such as improvements of productivity and physical specificities of the articles have been raised more and more.
Although the carbonaceous materials are excellent in physical properties as a material, for instance, corrosion-resistance, electric conductivity, mechanical strength, etc., in order to make the most of these excellent physical properties still effectively, a development of the composite carbonaceous materials prepared by combining and joining carbonaceous materials which are the same or different from each other in quality has been promoted. Hitherto, such composite carbonaceous materials have been used as the carbon product prepared by only adhering a plurality of the materials with an adhesive, and there are problems in such products concerning corrosion resistance, electric conductivity, dimensional stability, etc.
In recent years, a process has been devised for solving the above-mentioned problems by adhering the carbonaceous materials togehher with an adhesive and calcining the thus adhered materials in an inert atmosphere.
However, in the case of producing the composite carbonaceous products by such a process there are many occasions of exfoliation of the carbonaceous materials at the joining surface to each other and of forming cracks in the product during the process by the difference of the coefficient of thermal expansion- and contraction at calcination between the carbonaceous materials, thereby causing the reduction of the productive yield.
Further, as the demands for relatively large-sized carbon products and/or carbon products of complicated shape have been raised in recent years from the constructional and functional view points, the above-mentioned problems of exfoliation and cracks become to be particularly serious in the production steps on such occasions.
As has been shown above, on the composite carbonaceous materials, the following strict and heavy demands have been imposed.
Namely, in the production thereof, no exfoliation of the carbonaceous materials to each other is caused and no crack is formed in the product, and in the same time, the final product must retain the excellent properties originally possessed by the carbonaceous materials themselves, for instance, mechanical strength, electrical specificities, etc. Namely, there are many difficulties in the production of the composite carbonaceous materials.
In addition, fuel cells of the bipolar separator-type which use the bipolar separator(s) obtained by rib-processing the impermeable thin plate of graphite have been publicly known. On the other hand, the electrode substrate of monopolar-type which has been provided with ribs on one of the surfaces thereof and has the other surface which is a flat electrode surface, in which gaseous reactants permeates from the ribbed surface to the flat electrode surface has been developed. Further on the other hand, as the process for producing the conventional electrode substrate for monopolar type fuel cell, for instance a process wherein short carbon fibers are press-molded has been proposed (refer to U.S. Pat. No. 4,506,028).
These electrode substrates obtained by the conventional process consist of one layer of a wholly uniform construction. Since in such an electrode substrate consisting of a single and uniform layer, in the case where the bulk density is large, the diffusion coefficient of the gaseous reactant is small, the threshold current density is small and in the same time, the retaining amount of the electrolyte solution is small, and accordingly, the reduction of the performance occurs early, namely it has a demerit of short life of the fuel cell. On the other hand, in the case where the bulk density is small, there is a demerit that the mechanical strength such as bending strength thereof is low.
The present inventors have offered an electrode substrate having excellent physical properties, prepared from short carbon fibers as the basic material, wherein the passage of the gaseous reactant is formed near the center of the thickness of the porous carbonaceous layer as the gas-diffusion layer not by mechanical processing but by easier press-molding and heating treatment, the electrode substrate and the separator having been made to be one body in the carbonized state (refer to U.S. Pat. No. 4,522,895). By the just-mentioned offering, it has become possible to use an electrode substrate of a large gas diffusion coefficient, in other words, an electrode substrate having a gas-diffusing part of a small bulk density. In this type of electrode substrate, since the separator and the electrode substrate are integrated as one carbon body the contact resistance can be reduced sharply as compared to that in the conventional monopolar- and bipolar type electrode substrate. In the above-mentioned electrode substrate offered by the present inventors, the desirable elongated gas holes could be formed in the porous carbonaceous layer not by the rib-processing and boring but by the process of forming the porous carbonaceous layer from short carbon fibers as the basic material while using a binder of thermosetting resin having a specified carbonization yield and a micropore regulator having a specified granular size and thermally decomposing at a temperature of higher than the molding temperature.
However, in the step of carbonizing and calcining of the molded bodies in the process of producing the electrode substrate, the exfoliation of the porous carbonaceous layer and the gas-impermeable layer (the dense carbonaceous layer) to each other was inevitable, particularly in the large-type electrode substrate having the large surface of the substrate in spite of modifying the method of elevating the temperature of the molded materials to the temperature of calcination, resulting in the low production yield. Accordingly, fundamental improvement has been desired.
The reason why the exfoliation occurs in the step of calcination (to the maximum temperature of 3000.degree. C.) lies in the difference of the thermal expansion coefficient between the porous carbonaceous layer and the gas-impermeable layer (or the separator) in the heating step or in the difference of thermal contraction coefficient between both layers in the cooling step to room temperature after completing the calcination.
Accordingly, in order to offer the carbon product as the electrode substrate for fuel cells which solves the above-mentioned demerit, the difference of the thermal expansion and the thermal contraction coefficients at calcination between both layers is to be reduced or eliminated by interposition of a buffering layer which exhibits a function of absorbing the above-mentioned thermal expansion and contraction between both layers.
As the buffering material, the present inventors have aimed at a commercialized flexible graphite sheet which has relatively large coefficients of thermal expansion and contraction, an adhesive property to an adhesive and a relatively low permeability to gases. The flexible graphite sheet is obtained by press-molding of the so-called, expanded graphite particles prepared by acid-treatment of natural graphite, thereby expanding the interlayer of carbon-carbon bondings. The surface of the thus obtained flexible graphite sheet is scaly, and is somewhat gas-permeable, and accordingly, it has an adhesive property because of the possibility of impregnation of an adhesive. Since the used graphite sheet is flexible, it is well suitable for absorption of the thermal expansion and contraction.
As a result of studying the method for mutually joining the carbonaceous materials in the purpose of producing the composite carbonaceous products, the present inventors have found that (1) by interposing the above-mentioned flexible graphite sheet between the porous carbonaceous layer and the carbonaceous material for the separator and adhering the flexible graphite sheet to the porous carbonaceous layer and to the carbonaceous material for the separator via a carbonizable adhesive, the interlayer exfoliation which has hitherto been a large problem can be prevented and the production of the large-type carbonaceous electrode substrate becomes possible, and (2) by (a) processing the electrode carbonaceous layer into the protuberant parts and the flat plate part and joining the protuberant parts to the carbonaceous materials for the separator via the flexible graphite sheet, or (b) joining the flexible graphite sheets onto both of the surfaces of the carbonaceous material for the separator and after providing the carbonaceous protuberant parts on outer surfaces of the thus joined flexible graphite sheets and joining the thus provided protuberant parts onto the carbonaceous flat plate parts, thereby forming the passage of the gaseous reactants, it becomes possible to make the whole product thinner, and to reduce the electric- and thermal resistances, and further it becomes possible to obtain the composite carbonaceous products in large sizes and the composite carbonaceous products for electrode substrate having less demerits such as curvature, cracks and exfoliation because of the dispersion of the stress at the time of calcination, the composite carbonaceous product have been formed in one body as carbon, and based on the above-mentioned findings, the present inventors have attained the present invention.
Namely, the objective of the present invention is to provide a carbon product (a composite carbonaceous product) as the electrode substrate for fuel cells, comprising a carbonaceous material for a separator, two flexible graphite sheets provided on the both surfaces of the carbonaceous material, a plurality of carbonaceous protuberances provided on each outer surface of the two flexible graphite sheets and a pair of porous carbonaceous flat plates provided on each outermost surface of the product, the whole materials of the product having been integrated in one body as carbon by calcination in an inert atmosphere, the passage for the gaseous reactants having been formed by the combination of (1) the above-mentioned flexible graphite sheet or the flexible graphite sheet and the carbonaceous material for the separator, (2) the plurality of the carbonaceous protuberances and (3) the porous carbonaceous flat plate, the ratio (Sr/Se) of the total area of the cross section of the above-mentioned plurality of carbonaceous protuberances (Sr), which cross section is parallel to the surface of the above-mentioned carbonaceous material for the above-mentioned separator, to the total area of the joining surface of the porous carbonaceous flat plate (Se) being from 0.2 to 0.8 and the space between the two neighbouring carbonaceous protuberances (d) (refer to FIG. 3) being not more than 10 mm.
Further, the objective of the present invention is to provide a large-type carbonaceous composite material which has excellent specific properties and is free from curvature, crack and exfoliation as the product.
Furthermore, the objective of the present invention is also to provide a process for production of a large-type carbonaceous composite material which does not exhibit the demerits due to the conventional techniques and particularly, is free from curvature, crack and exfoliation.